1. Field of the Invention
The present invention relates to a fixture for implantation and osseointegration with bone tissue whose purpose is to carry a prosthesis, a prosthesis anchoring system, and a prosthesis attached to the fixture.
2. Description of the Related Art
It is known to implant fixtures in a person""s bone tissue in order to attach different types of prostheses to them, dental prostheses for example.
To make certain that the fixture anchorages are permanent, a number of factors must be fulfilled with regard to, among other things, material selections and operation techniques. In practice, fixtures developed by Professor Brxc3xa5nemark made of pure titanium with micro-pitted surfaces have shown very good long-term anchoring abilities. The screw-shaped fixtures are operated into a bone, the jawbone for example, and are permitted to heal inxe2x80x94osseointegratexe2x80x94for a certain time period, usually a few months. Thereafter, a prosthesis can be mounted on the fixtures.
Most types of these fixtures are provided with external threadsxe2x80x94and are thus implanted screwsxe2x80x94and they are attached by screwing them into holes made in the bone tissue. The holes are often pre-threaded, but it is also possible to screw in self-tapping screws. Normally, the fixtures require shorter or longer times for healing-in before they should be exposed to significant loads. On the fixtures, one can attach different types of prostheses, and for dental prostheses one can attach individual teeth, i.e., one tooth per fixture, but it is also possible for two or more fixtures to serve as attachment points for a bridge structure containing a number of teeth or even an entire row of teeth. The fixtures are therefore designed in a suitable way at their free ends.
The free end of such a fixture to which a prosthesis is to be fastened, i.e., the fastening end, is equipped with some form of fastening device that will engage a corresponding fastening device on the prosthesis. It is herewith usual to have the attachment arranged so that a bearing surface on the prosthesis or a prosthesis-carrying element will be in contact with a supporting surface on the fixture""s fastening end. The supporting and bearing surfaces should be parallel to ensure fully satisfactory functionality. This seldom poses any problem for prostheses of the type that are attached by means of only one fixture since in such cases the prosthesis, which is usually fastened by a threaded joint to the fixture, will have its bearing surface forced into contact with the fixturets supporting surface throughout its entire area when the prosthesis is screwed into place.
In many cases, a prosthesis can be anchored using more than one fixture. This is especially true in connection with jaw reconstruction when a prosthesis containing a row of teeth is to be attached, but it can also occur for prostheses used for other parts of the body.
When such a prosthesis or a holder for such a prosthesis is to be attached to two or more fixtures, the fixture onto which the prosthesis is first attached and screwed into place will force the prosthesis into a certain position as a result of the fact that its bearing surface, which is pressed against the fixture""s supporting surface will forcibly have its direction determined by said supporting surface. For screwing a prosthesis to the second fixture and to additional fixtures if any, it is desirable that the corresponding bearing and supporting surfaces on this/these fixture(s) also be parallel so that there will be no misalignment. When anchoring fixtures in bone tissue one tries, of course, to have the supporting surfaces assume their intended directions relative to each other with the greatest possible precision, and these directions should be adapted to the corresponding bearing surfaces on the prosthesis. Usually the fixture surfaces are to lie in the same plane or at least be parallel.
To fully achieve this is very difficult, and it must be expected that an anchored fixture will deviate directionally from what is intended. Anchoring two or more fixtures fully parallel is also very difficult, and the possibility of obtaining precisely matching heights is remote. A deviation of even a mere degree or so results in a deficient fit between the supporting surfaces and bearing surfaces when a prosthesis, as described above, is attached to the fixtures. Since the prosthesis element or prosthesis holder is generally made from a completely rigid material such as stainless steel, a faulty fit cannot be compensated for by deforming the element or holder. Instead, there is poor contact with one or more of the fixtures"" supporting surfaces and strains develop in the prosthesis structure when one tightens the threaded joint to attach the prosthesis to the fixtures. Moreover, these strains are propagated down into the bone tissue. This can cause discomfort and trouble for the patient while detracting from the functionality of the prosthesis and shortening its life expectancy.
This problem is especially common in connection with jaw reconstruction where a superstructure with a dental prosthesis is screwed in place on the fixtures directly or via an attachment bar. Such a lack of parallelism among the fixtures poses a major problem, particularly in connection with the use of prefabricated superstructures not intended for extensive individual adaptation. In order to compensate for this non-accuracy it is earlier known e.g. from EP 0126 624, EP 0370 590 and EP 0466 267 to provide an intermediate elastic device between the fixture and the prosthesis.
The present invention eliminates the disadvantages described above that are encountered when the directions of the fixtures and/or their height positions do not fully match what was intended, and without inserting special compensation elements.
In accordance with the invention, this has been achieved by means of a fixture for implantation and osseointegration with bone tissue whose purpose is to carry a prosthesis, by means of a prosthesis anchoring device having a prosthesis base that engages the fixture, and by means of a prosthesis containing a number of attachment units arranged integrally with the prosthesis or with a separate prosthesis base, where each attachment unit is arranged to be attached to a fixture anchored in a person""s bone tissue, and also containing a bearing surface arranged so that when the prosthesis is attached to said fixtures, the bearing surface will be in contact with a supporting surface on the appropriate fixture, the bearing surface and the supporting surface both being substantially perpendicular to a center axis of a related fixture wherein at least one of said bearing surfaces is provided with a deformation device that is deformable in at least one direction that is perpendicular to the bearing surface to attain absolute parallelity between the supporting surface and the bearing surface when the bearing surface is pressed against the supporting surface.
Because the supporting surface on the fixture or the bearing surface on the prosthesis is integrally provided with a deformation device that can be deformed plastically in a direction perpendicular to the respective surface when misalignment occurs, the deformation device will be deformed to a corresponding extent. As a result, the bearing surface will have a fully functional contact with the supporting surface in spite of the misalignment. Since angular misalignment is absorbed by such deformation devices, the tightening of the retainer screws will not implement any strains in the patient""s bone tissue.
The fixture is preferably designed with at least parts that are rotationally-symmetrical, wherewith its supporting surface is perpendicular to its center axis.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.
In one preferable embodiment, the deformation device is designed as an axially aligned flange, suitably cylindrical and coaxial with the center axis.
It is preferred that the flange be peripherally arranged on a circular supporting surface to provide optimal deformation distribution. This will occur as a result of the fact that the flange will then be located at as long a distance as possible from the center axis.
In other preferred embodiments, the flange is provided with slots and/or is beveled axially outward, thereby facilitating deformation.
Moreover, it is preferred that the flange""s outer end be pointed, i.e. sharp, thus providing progressive resistance to deformation.
The above and other preferred embodiments of the invention are set forth in the dependent claims.